The present invention arose in the coal mining equipment industry.
As schematically shown in FIG. 1, when a coal-carrying conveyor belt rips transversally or separates at a seam, usually the severed ends move away from one another several feet in the moments of trauma following the accident.
In overcoming the problem represented by a broken belt laden with coal, generally it is essential that an in situ repair be undertaken and performed, even if the repair will be good only long enough to let the coal on the belt be conveyed out. Then, a replacement or more permanent repair can be made under less stressful and awkward conditions. Sometimes it is possible to make the in situ repair a permanent one.
One of the important initial tasks when undertaking an in situ repair is to bring the spread-apart severed ends of the belt back into alignment and abutment or adjacency.
In order to accomplish this task a succession of decreasingly make-shift clamp devices have been devised.
In the classical prior art, which continues to be practiced in many mines even today, adjacent each broken end B1, B2 of the belt B, the respective belt end is sandwiched between an upper timber TU and a lower timber TL oriented one over the other crosswise to the run of the belt. The timbers are longer than the belt is broad, so the ends of each timber extend beyond the belt edges. Laterally outwardly of each side of the belt, at each broken end B1, B2, securement means S, such as studs or cap screws are vertically installed through superimposed holes through the respective timbers. These are fastened and made tight with the aid of respective nuts and flat washers, to provide full width clamps C1, C2, for the respective belt ends B1, B2.
To prevent slippage of the timbers of the clamps, cap wedges CW are driven between the timber and the belt, as needed. (Cap wedges are generally on hand anyway for use at the top of mine roof support timbers.)
Clamp C1 is then lashed to Clamp C2 by tensile means T1, T2 such as chains, cables, block and tackle, etc. and one is pulled toward the other using pulling means W, e.g. a portable winch or hoist, to or mobile equipment which can pull on the tensile means T1, T2. Sometimes both tensile means T1, T2 are attached to the same pulling device, and other times one pulling device is pulling on T1 in one direction and another is pulling on T2 in the other.
The beauty of the above arrangement is that usually, when a belt breaks, all the parts needed to make and install the clamps C1 and C2, and the pulling means can be cobbled together from whatever is close at hand.
However, especially in a low seam in an underground mine, where space is at a premium it is awkward trying to assemble, install and use such full width clamps made of timber and hardware. Other, made for the purpose, full width clamps are fine for making above ground repairs, but are at least as awkward to use in low seam coal. Not only are they cumbersome to use because of their bulk, but they have to be carried to the site of the break, and their bulk and weight are against them for this reason also.
In response to the need for something more convenient to carry and use, edge clamp sets were developed, with four individual clamps to a set. A prior art clamp of this type is shown by itself in FIG. 2, and the set is shown installed and in use in FIG. 3. Each clamp CL1, CR1, CL2, CR2 has a set of jaws JU, JL which grip a respective marginal edge portion of the belts near the respective belt broken ends.
Each of these clamps has an anchor point A to which the respective chain T end attaches for the pulling operation. Although these prior art edge clamps have been well received, some users have complained that all too often, when a sufficient tension is pulled on the chains to get the broken belt ends moving back toward one another, one or more of marginal edge portions of the belt will rip next to its edge clamp. The relative location where a rip generally developes is indicated next to the clamp C1L by the line R.
The present inventor has studied the prior art clamp of FIGS. 2 and 3, and believes he has discovered what causes the belt rips to occur. His reasoning is presented graphically in FIG. 4. In brief, in this prior art clamp, the site where the jaw set JU/JL grips the respective marginal edge of the belt is laterally offset from the anchor point A of the clamp. So that when the chain is attached for pulling the resulting torque tends to concentrate a ripping force at R. Clearly, the shorter the jaw-contact "footprint" is in the lateral direction D, the less will be the ripping torque for any given pulling force on the anchor point A of the respective clamp. For this reason, the ideal prior art clamp of this type has zero length jaws. In the actual clamps of this type, the provision of jaws of a finite length is a compromise from zero length, which walks a fine line between being so long as to cause belt twisting and tearing and being so short as to have insufficient clamping area to apply the clamping force without generating excessive compressive stress in the belt under load.
A clamp that is a particular variation of the one shown in FIGS. 2 and 3 is shown in the prior U.S. Pat. No. of Travis, 3,955,810, issued May 11, 1976.